As an example of a linear motion rolling guide, there is known a "linear guideway" [see, for example, Eiichi Watabayashi, "JIS How to Use Series--Rolling Bearing Manual," 1st Edition, 1st Print, Japan Standard Association (a foundation), (Feb. 20, 1999), pp. 299-301]. The linear guideway is provided with an elongate guide rail having axial ball rolling grooves formed on both side faces of the guide rail, a carriage mounted movably over the guide rail, the carriage having ball rolling grooves formed on inner side faces thereof in an opposed relation to the ball rolling grooves of the guide rail and also having endless circulation paths which include the ball rolling grooves of the carriage, and a plurality of balls adapted to roll in the endless circulation paths of the carriage and bear a load in both ball rolling grooves.
As the balls in the above linear guideway there mainly are used steel balls (JIS B 1501) for a ball bearing which balls are available commercially. However, the following problems have been encountered heretofore:
(1) With rolling of steel balls positioned between the ball rolling grooves of the carriage and the like grooves of the guide rail, which area is a load-bearing area in the endless steel ball circulation paths, adjacent steel balls may sometimes come into contact with each other. Since the steel balls rotate in the same direction, there occur opposite rotations at the portion of strong contact between adjacent steel balls and the resulting force acts to obstruct smooth rolling of each steel ball. Further, a slip force is also generated at the contact portion. If the carriage slides in such a state, each steel ball will repeat slip and rolling, or repeat collision with adjacent steel balls or with the ball rolling grooves, giving rise to a noise or vibration of a relatively high frequency or an early-stage wear, with consequent deterioration of the guiding accuracy and service life required as a linear guideway. Particularly, there is a recent tendency for the linear guideway to be used at a higher speed, and thus the above point has been noted as a problem to be solved. PA1 (2) Even if a predetermined number of steel balls, which number is determined in design, are loaded into each endless circulation path, a gap corresponding to half of a steel ball or one steel ball is formed in the same path, although this depends, for example, on the overall length of the path. Consequently, when a lightly pre-loaded linear guideway is mounted vertically, upper steel balls may drop and strike against lower steel balls and generate an offensive metallic sound. Further, the collision between steel balls may result in formation of very small dents on their spherical surfaces, which dents may impair the guiding accuracy required as a linear guideway. Pre-loading for the linear guideway is usually effected by an oversized ball method. PA1 (1) Disposing a rubber or soft plastic spacer ball between adjacent steel balls in each endless circulation path, the spacer ball having a diameter slightly smaller than the diameter of each steel ball, as described in Japanese Published Unexamined Utility Model Application No. 180015/1989. PA1 (2) Disposing a plastic spacer between adjacent steel balls in each endless circulation path, the plastic spacer being smaller than the diameter of each steel ball, as described in Japanese Published Unexamined Patent Application 173946/1994 and 281154/1998. PA1 (3) Using a ball chain to retain steel balls in each endless circulation path, as described in Japanese Published Unexaminer Patent Application No. 52217/1993. PA1 (1) In the case of using spacer balls, a load carrying capacity of the linear guideway decreases. For example, if the steel ball-spacer ball arranging ratio is set at 1:1, the value of the load carrying capacity of the linear guideway is reduced by half, so the adoption of such spacer balls has actually been impossible. PA1 (2) In the case of using such a spacer as described in the foregoing Japanese Published Unexamined Patent Application No. 173946/1994, it has been impossible to load both steel balls and spacers closely, leaving no space, into each endless circulation path, although this depends on the overall length of the same path and is attributable to a processing error for the spacer thickness. The gap thus formed has sometimes resulted in steel balls coming off concave faces formed at both ends of each spacer, with consequent occurrence of a lock phenomenon. This lock phenomenon has sometimes caused breakage of the spacer. PA1 (3) Using a ball chain PA1 1 Since the balls do not contact with one another, it is possible to prevent the occurrence of noise and vibration of a relatively high frequency caused by mutual contact of balls or prevent an early-stage wear of the balls and hence possible to maintain the guiding accuracy and service life required as a linear guideway. PA1 2 When the linear guideway is mounted vertically in a lightly pre-loaded state, there is no fear that an upper ball may drop and strike against a lower ball. Thus, the generation of an offensive noise caused by collision is prevented. Besides, since a very small dent is not formed on the spherical surface of each ball, it is possible to ensure a stable guiding accuracy required as a linear guideway. PA1 3 Unlike the case where only balls and spacer balls are used, there is no fear of decrease in the load carrying capacity of the linear guideway. PA1 4 Since it is impossible that each ball will become disengaged from the concave faces formed at end portions of adjacent spacers, it is possible to prevent the occurrence of a lock phenomenon and breakage of the spacers caused by such lock phenomenon, thus making it possible to ensure smooth rolling of each ball. PA1 5 Since the spacers are independent and not connected with one another, such an extra machining as in the use of a ball chain is not needed for the endless circulation paths of the carriage. Moreover, it is possible to cope with various products without being restricted by the length of the carriage.
The following three means have been proposed for solving the above-mentioned problems:
However, even the above three means have involved the following problems:
Steel balls and spacers may be arranged closely without leaving any space by making the spacers different in thickness, as described in the foregoing Japanese Published Unexamined Patent Application No. 281154/1998. However, since plastic spacers are formed by injection molding with use of a mold, the formation of spacers having various thicknesses leads to a considerable increase of cost. Thus, it has so far been difficult to realize such idea.
1 It is necessary that a recess for receiving connector members of the ball chain therein be formed on an inner wall surface of each endless circulation path, thus requiring an extra cost for machining. PA2 2 It is necessary that steel balls be inserted into a mold and be subjected to injection molding integrally with spacers and connector members. However, for obtaining ball chains of desired lengths, it is required to fabricate various long molds to match the diameters of steel balls used. Thus, even a mere fabrication of the molds requires a considerable cost. PA2 3 Linear guideways, even of the same nominal model number, include two types of carriage lengths--standard type and long type--. Therefore, it is necessary to fabricate ball chains of two different lengths.